When separate parts are to be connected together, a typical procedure involves bolting or riveting the parts together. Some sort of bolting device is usually fitted through a hole in each part to bolt the parts together to form a joint. There are many applications for these joints, such as in aircraft and in other vehicles. During operation, aircraft and other vehicles are subjected to oscillatory or dynamic loading. Such oscillatory loading produces stress on the joint and if the amount of this stress and the number of oscillatory cycles of stress are great enough, fatiguing of the joint will result. Such fatiguing is exhibited as cracks or breaks in the joint.
Various patents have disclosed methods of increasing the fatigue life of joints. U.S. Pat. No. 4,164,807 discloses a method of forming a coldworked joint. The patent describes placing a seamless tubular member or bushing within a hole through a work piece where the tubular member has a prescribed hoop strength sufficiently to allow the seamless tubular member to be radially and circumferentially expanded a first prescribed amount, and a prescribed column strength sufficient to substantially prevent axial movement of the outside of the tubular movement as the tubular member is expanded through the hole. The tubular member is held within the hole so that it is substantially axially fixed with respect to the hole. An expansion mandrel, having an expansion section thereon with a maximum diameter a second prescribed amount greater than the inside diameter of the tubular member, is passed through the tubular member to expand the tubular member radially and circumferentially sufficiently to induce a compressive stress gradient of a prescribed magnitude in the material of the work piece substantially uniformly about the entire surface of the hole. The inducement of this compressive stress gradient increases the fatigue strength of the hole.
The method described above with reference to the patent has limitations. Microscopic metal on metal friction related stress, commonly referred to as fretting, between the tubular member and the walls of the hole reduces fatigue strength. Although coldworking the hole as described above increases the fatigue strength, fretting limits the increase in the fatigue strength. The fatigue strength of parts which exhibit fretting is historically less predictable than without fretting. Another disadvantage with the method is that the tubular member is difficult to replace during a repair or overhaul operation. Due to the high interference fit between the tubular member and the walls of the hole, pressing out the tubular member is not recommended as this would damage the walls of the hole and the work piece. Furthermore, machining out the tubular member is not suitable since this operation could also damage the walls of the hole.
Therefore, it is an object of this invention to reduce fretting thereby increasing the fatigue life of the joint. Furthermore, it is an object of this invention to allow the tubular member (hereinafter referred to as the bushing) to be easily replaced without damage to the hole, during a repair or overhaul operation.